Department of Chemistry & Physics, Georgia College Andstateuniversity

Linkage Isomerism

Kevin F. Dunn

Department of Chemistry & Physics, Georgia College andStateUniversity,

Milledgeville, GA31061

November 24, 2008

Inorganic Laboratory Fall 2008

Abstract

This experiment involved the exploration of the structural isomerism of the NO2- ligand. Two equivalent structural isomers were prepared except that one ligandbonded to the cobalt atom via an oxygen atom while the other bonded via the nitrogen atom. The structures of each isomer, nitropentamminecobalt (III), and nitritopentamminecobalt (III), were distinguished using IR spectroscopy. It was determined that the nitro isomer was thermodynamically more stable and the nitrito isomer was the kinetic product.

Introduction

In inorganic chemistry, coordination complexes describe compounds composed of a metal atom or ion to which one or more ligands donates electrons.1 This can give rise to structural isomerism which arises when atoms of coordinating groups can ligate to an atom of a coordination complex is more than one way.

Many ligands are ambidentate, meaning they have the ability to bond a central atom in more then one way. Examples of this include NCS-, which can coordinate via the N or S atom and NO2- which can coordinate via the N or O atom. In this particular experiment, the NO2- ligand is explored and can form a coordination complex with cobalt through the N atom or O atom to yield two structural isomers, a nitrito or the more stable nitro.2

Figure 1: The two ways that NO2- can form coordination complexes to Co.

The coordination complex pentamminemonochloro cobalt (III) chloride, [Co(NH3)5Cl]Cl2, forms two linkage isomers in the presence of the NO2- ligand. This complex is generally synthesized by exposing the cobalt ion to ammonium hydroxide and then acidifying with HCl.2 When this complex is exposed to NO2-, two linkage isomers result, nitropentamminecobalt (III) and nitritopentamminecobalt (III).

Figure 2: Preparation of the coordination complex.

Figure 3: Preparation of the nitrito isomer.

Figure 4: Preparation of the nitro isomer.

In the nitrito complex, metal-to-ligand bonding is through one of the oxygen atoms while the metal-to-ligand bonding for the nitro complex takes place through the nitrogen.3 The nitro complex is the thermodynamically stable product while the nitrito complex is the kinetic product and over time will transform into the more stable thermodynamic product.

In analysis, the two complexes are distinguishable from one another based upon their individual IR spectrum. The nitro complex will give a significantly different peak than that of the nitrito complex due to the differences in the bonding of the two structures.

Experimental2

Cobalt(II) carbonate (10.0 g) was first added slowly to a solution consisting of ammonium chloride (5.0 g) with concentrated ammonia (30.0 mL). While stirring 30% hydrogen peroxide (8.0 mL) was added slowly, upon the addition a brown slurry was formed. After the reaction has ceased concentrated HCl (30.0 mL) was added slowly to the slurry while heating to 85°C. The mixture was allowed to heat and stir for twenty minutes before being cooled to room temperature. The solution was then filtered. The resulting crystals were washed with cold water (20.0 mL) and 6.0M HCl (20.0 mL). The product was then allowed to dry overnight in an oven at 100°C.

To obtain the first isomer, [Co(NH3)5Cl]Cl2 (1.5 g) was dissolved in water (15.0 mL) which concentrated ammonia (5.0 mL) had been added. This was warmed on the water bath, filtered, cooled, and then acidified with dilute HCl. NaNO2 (2.0 g) was added and the solution was heated gently until a red precipitate first formed dissolved. The solution was colled and HCl (20.0 mL) was added before the solution was cooled in ice. The organe crystals were filtered off and then washed with alcohol and ether.

In obtaining the second isomer, [Co(NH3)5Cl]Cl2 (1.5 g) was dissolved in water (25.0 mL) which concentrated ammonia (5.0 mL) had been added. This solution was filtered and HCl was added dropwise (6.0 mol dm-3) until the solution was neutral. Then NaNO2 (1.5 g) was added to the solution followed by HCl (1.5 mL). This solution was allowed to stand in ice before the pinkish product was filtered off and washed with ice water, alcohol, and ether before being allowed to dry.

Results/Discussion

A UV/Vis spectrum was obtained for the product [Co(NH3)5Cl]Cl2 before the synthesis of the two individual isomers. The spectrum for this compound indicates that it was successfully synthesized.

Figure 5: UV/Vis spectrum for [Co(NH3)5Cl]Cl2.

Due to complications with the IR machine and running a solid state IR, the spectra for the two isomers showed a significant amount of interference from water and carbon dioxide which made reading the spectra virtually impossible. As a result, the spectra for this particular experiment was obtained from the Fall 2007 class and then analyzed. The two structural isomers can be distinguished from one another by comparing their individual spectrum.

Figure 6: IR spectrum for complex 1.

Figure 7: IR spectrum of complex 2.

Both spectra contained peaks around 850 cm-1, 1320 cm-1, and 1600 cm-1. According to the literature for these complexes, the peaks have been identified as values for amine ligands, specifically the NH3 ligand.4 Since both complexes contain this ligand as well as the NO2- ligand, these peaks are to be expected in both spectra. The peaks at 3250 cm-1 and 1550 cm-1 are indicative of N-H stretching and bending. The remaining peaks in the spectra for the two compounds distinguish whether the metal-to-ligand bonding is through the oxygen atom or nitrogen atom of the NO2- ligand.

In the nitrito complex, absorption bands between 1400 and 1550 cm-1 as well as those between 900 and 1300 cm-1 can be attributed to the ONO stretching. On the other hand, the nitro complex has distinct absorption bands from 825 to 1450 cm-1. This is due to the nitro ligand stretching. Also note in the spectra of each complex that the decrease in these bands is characteristic of the kinetically favored nitrito complex.4

An IR spectrum was obtained of the nitrito complex several days later and this spectrum was nearly identical to the nitro complex. This is explained by the natural degradation of the kinetic product, nitrito, into the thermodynamically stable product, nitro.

Figure 8: IR spectrum of complex 1 several days later.

Conclusion

Analysis of the IR spectra of the two isomers indicates that nitropentamminecobalt (III) and nitritopentamminecobalt (III) were successfully synthesized. The nitrito complex is the kinetically favored product but over time this complex degrades naturally into the more thermodynamically stable product which is the nitro complex.

Acknowledgments

I would like to acknowledge my laboratory partners Joseph Tamasi and Claudia Ramirez as well as Emily Williams, Leah Corley, Yuri Blake, and Tiffany Shoham for assisting with the experiment. I would also like to acknowledge Candace Rowell for providing the procedure as well as the spectra used in the experiment. Many thanks are also due to the Department of Chemistry here at GeorgiaCollege and StateUniversity for providing the necessary resources to carry out the experiment.

References

1)Miessler, Gary L., Tarr, Donald A. Inorganic Chemistry. Third Edition. Pearson Education Inc., Upper Saddle River, New Jersey

2)Tanaka, John., Suib, Steven L. Experimental Methods in Inorganic Chemistry. Prentice Hall, Upper saddle River, New Jersey

3)J. Chem. Ed. 1974, 51, 553.

4)J. Amer. Chem. Soc. 1956, 78, 887.